Countershaft transmissions having a plurality of speed ratios in each direction of operation are particularly useful in the drive line of heavy duty work vehicles such as wheel loaders, log skidders, lift trucks, and earthmoving machines. These transmissions are advantageous in that a plurality of rotating disc type clutches and associated continually meshing gears can be so positioned on the usual parallel shafts as to allow considerable design flexibility and an adaptability to the elevational drop requirement between the input and output axes.
One of the problems with such countershaft transmissions is that they have required too many shafts or gears. For example, many of them have an extra idler gear in order to provide the reversing function. This requires an additional bearing system along another axis parallel to that of the primary shafts at increased expense.
Another problem with these prior countershaft transmissions is that they have been axially too long. For example, two or even more rotating clutch assemblies and a plurality of gears have been arranged along a single shaft axis in such a way with respect to the supporting walls of the housing as to be of excessive length. Simultaneously, the shafts, clutches and gears must be closely radially spaced and internested so as to provide both a radially compact and axially compact module.
One countershaft transmission of the type generally described that has many advantages is disclosed in International Application No. PCT/US84/01352 filed Aug. 24, 1984 and published under the Patent Cooperation Treaty (PCT) on Dec. 19, 1985 as Publication No. W085/05665, and which is assigned to the Assignee of the present invention. Among other things it features a minimum elevational clearance above the upper input axis. Specifically, the input shaft is normally connected to the output member of the engine and/or the optional torque converter assembly of the vehicle, and the construction is such as to avoid locating large diameter components such as a rotating clutch thereat. Thus, the floor plates or portions of the operator station above the input shaft need not be unnecessarily elevated.
Furthermore, the individual speed reduction ratios of the desired transmission should not only provide the range of output speeds that will best serve the vehicle requirements, but should be capable of convenient modification. Specifically, it is preferred that for one track-type tractor application, each of the output shaft speeds corresponding to a reverse travel direction of the vehicle should be over 15% faster than the corresponding forward speed in the same gear. It should be an easy matter to change this relative speed, between first forward and first reverse, for example, without requiring modification of the individual speeds steps, without changing the central axes of the shafts, and without requiring much more than the substitution of certain gear pairs within the countershaft transmission. The countershaft transmission illustrated in PCT Publication No. W085/05665, and mentioned above, does not have a construction that will conveniently meet these requirements.
In certain applications, it is also desirable that the number of forward speeds of the countershaft transmission be easily increased to enlarge the capability thereof. For instance, a three forward speed unit used in conjunction with a hydrodynamic torque converter could be used in a track-type tractor for bulldozing and earthmoving tasks involving relative wide fluctuations of the output torque levels. On the other hand, a six forward speed unit serially connected to an engine without such a torque converter would be valuable in an agricultural tractor wherein the draw bar loads are more nearly constant and the greater efficiency of a direct mechanical drive would result in lower operating costs.
Still other desirable features are to maximize parts commonality by using similarly sized rotating clutch elements, to provide a long service life by so constructing and arranging the gears that undesirably high operating speeds or clutch plate engagement speeds are avoided, and to place the individual members thereof in locations that will allow their convenient assembly or disassembly with respect to the housing or case elements.
Accordingly, what is needed is a structurally simple, and both radially and axially compact constant mesh countershaft transmission providing a plurality of forward speeds using a minimal number of shafts, rotating clutches and associated elements. Preferably, the transmission should provide three or more forward speeds, should have a long service life using disc type rotating clutches and appropriately interconnected constantly meshing gears, and should be so constructed and arranged as to allow the effective conversion of the speed ratio collectively between the forward and reverse speeds independently of the individual speed reduction ratio steps or vice versa. Furthermore, it should be easy to increase the number of forward speed ratios with but minor modification, such as by the addition of one more clutch and a gear. This can provide a family of transmissions that can maximize parts commonality while making the family adaptable to a wide variety of vehicular applications.